The skin of a human is a composite material of the epidermis and the dermis. The topmost part of the epidermis is the stratum corneum, which is the stiffest layer of the skin, as well as the one most affected by the surrounding environment. Below the stratum corneum is the internal portion of the epidermis. Below the epidermis, the topmost layer of the dermis is the papillary dermis, which is made of relatively loose connective tissues that define the micro-relief of the skin. The reticular dermis, disposed beneath the papillary dermis, is tight, connective tissue that is spatially organized. The reticular dermis is also associated with coarse wrinkles. At the bottom of the dermis lies the subcutaneous layer.
The skin's major functions include protection, excretion, secretion, absorption, thermoregulation, pigmentogenesis, accumulation, sensory perception, and regulation of immunological processes. These functions can be detrimentally affected by many structural changes in the skin, such as aging, disease, or exposure to solar radiation, pollution, and other factors present in the environment. The physiological changes generally associated with skin aging may include impairment of the barrier function and decreased turnover of epidermal cells.
Mechanical properties of the skin, for example elasticity, are controlled by the density and geometry of the network of collagen and elastic fiber tissue therein. Damaged collagen and elastin lose their contractile properties, resulting in skin wrinkling and skin surface roughness. As the skin ages or becomes unhealthy, it acquires sags, stretch marks, bumps, bruises or wrinkles. Further, it roughens, and its ability to synthesize Vitamin D decreases. Aged skin also becomes thinner and has a flattened dermoepidermal interface because of the alterations in collagen, elastin, and glycosaminoglycans.